1. Field of the Invention
The present invention relates to a phase shift mask, a method of manufacturing the phase shift mask and a method of forming a pattern using the phase shift mask. The invention more particularly relates to a phase shift mask provided with phase shift portions of Levenson type and Halftone type, a method of manufacturing the phase shift mask and a method of forming a pattern using the phase shift mask.
2. Description of the Background Art
As higher integration and miniaturization have been achieved in a semiconductor integrated circuit, miniaturization of a circuit pattern formed on a semiconductor substrate (hereinafter simply referred to as a wafer) has also been proceeded.
As a basic technique for generating the pattern, photolithography is widely known among others. Although various development and improvement have been made in the field, dimension of the pattern has still become smaller, and requirement for improved resolution of the pattern has also become stronger.
According to the photolithography technique, a mask (original) pattern is transferred to a photoresist coating a wafer, and underlying film to be etched is patterned using the photoresist. At the time of the transfer, the photoresist is developed. Through the development process, the photoresist of the type in which a portion exposed to light is removed is called a positive type, while the type in which a portion not exposed to light is removed is called a negative type photoresist.
Resolution limit R (nm) in the photolithography employing the demagnification exposure method is represented as EQU R=k.sub.1 .multidot..lambda./(NA)
where .lambda. is wavelength (nm) of the light used, NA is numerical aperture of a lens and k.sub.1 is a constant dependent on the resist process.
As can be appreciated from above equation, in order to improve the resolution limit R to obtain a fine pattern, the values K.sub.1 and .lambda. should be smaller, and the value NA should be larger. In other words, what is required is to reduce the constant dependent on the resist process as well as to shorten the wavelength and to increase NA. However, improvement of light source or the lens is technically difficult, and depth of focus .delta. (.delta.=k.sub.2 A .lambda./(NA).sup.2) of light might become shallower by shortening the wavelength and increasing NA, thus causing deterioration of the resolution.
In view of this, studies of miniaturization of the pattern by improving not the light source or the lens but the photomask are proceeded. Lately, a phase shift mask has been attracting much attention as a photomask allowing improvement of the resolution of the pattern. The structure and principle of such a phase shift mask will be hereinafter described in comparison with an ordinary photomask. The description below will be directed to a phase shift mask of the Levenson type and that of the Halftone type.
FIGS. 10A, 10B, and 10C respectively show a cross section of a mask, electric field on the mask, and light intensity on a wafer when an ordinary mask is used. With reference to FIG. 10A, the ordinary photomask is structured to have a metal mask pattern 103 formed on a glass substrate 101. The electric field on such an ordinary photomask is pulse modulated spacially by metal mask pattern 103 as shown in FIG. 10B.
Referring to FIG. 10C, if the pattern has smaller dimension, exposure light transmitted through the photomask extends into a non-exposed region (a region where transmission of the exposure light is blocked by metal mask pattern 103) on the wafer due to the diffraction effect of the light. The light is thus directed to a region not to be exposed on the wafer, resulting in deterioration of contrast of light (difference of light intensity between an exposed region and a non-exposed region on a wafer). The resolution is degraded and transfer of a fine pattern becomes difficult.
FIGS. 11A, 11B, and 11C respectively show a cross section of a mask, electric field on the mask, and light intensity on a wafer when a phase shift mask of Levenson type is used. With reference to FIG. 11A, an optical member called phase shifter 105 is provided on an ordinary photomask.
More specifically, chromium mask pattern 103 is formed on glass substrate 101 to provide an exposure region and a shading region, and phase shifter 105 is formed at every other exposure region. Phase shifter 105 has a function of shifting the phase of the transmitted light by 180.degree..
Referring to FIG. 11B, in the electric field on the mask generated by the light transmitted through the phase shift mask, the phases are alternately inverted by 180.degree. since phase shifters 105 are provided at every other exposure region. As described above, adjacent exposed regions have opposite phases of light, so that beams of light cancel each other out due to the interference of light in the portions where light beams of opposite phases are overlapped.
As a result, as shown in FIG. 11C, the intensity of the light becomes weak in the boundary portion between the exposed regions, then sufficient difference of light intensity between the exposed region and the non-exposed region on the wafer can be obtained. The improvement of the resolution is thus possible to allow the transfer of a fine pattern.
FIGS. 12A, 12B, and 12C respectively show a cross section of a mask, electric field on the mask, and light intensity on a wafer when a phase shift mask of the Halftone type is used. With reference to FIG. 12A first, the phase shift mask of the Halftone type is also provided with an optical member called phase shifter 106 as the phase shift mask of the Levenson type described above.
Optical member 106 is deposited on only a semi-transparent film 103 on glass substrate 101. Phase shifter 106 and semitransparent film 103 thus constitute a double layered structure. Phase shifter 106, as described above, has a function of shifting the phase of the transmitted light by 180.degree., and semitransparent film 103 has a function of attenuating the intensity of the exposure light without blocking it completely.
Referring to FIG. 12B, since the double layered structure of phase shifter 106 and semitransparent film 103 is provided, phases of the electric field on the mask are alternately inverted by 180.degree., and intensity of one phase becomes smaller than that of the other phase. As a result, the phase of light is shifted by 180.degree. by transmission through phase shifter 106, and the intensity of light is attenuated by transmission through semitransparent film 103 such that a prescribed thickness of the film of the photoresist is left after development. Phases of light are reversed at exposure regions adjacent to each other, so that light is canceled out at a portion where the light beams of opposite phases overlap.
As a result, as shown in FIG. 12C, light intensity can be reduced at an edge portion of the exposed pattern since phases are reversed at the edge portion of the exposed pattern. Accordingly, difference of the light intensity of the exposure light transmitted through semitransparent film 103 and not transmitted therethrough increases at corresponding regions, and resolution of a pattern image can be enhanced.
There are various types of the phase shift mask, including the Levenson type and the Halftone type. The phase shift mask of the Levenson type is known to be effective for a densely formed line/space pattern. The mask is also effective for producing an isolated dark line. More specifically, an isolated dark line of high resolution can be obtained by increasing width Ln and La of light transmitting regions and decreasing width Ls of the shading region sandwiched between the light transmitting regions in FIG. 11.
However, it is difficult to produce an isolated bright line using the phase shift mask of the Levenson type. In FIG. 11, an isolated bright line can be generated by increasing width Ls of the shading region and decreasing the width La (or Ln) of the transmitting region sandwiched between the shading regions. However, there is no interference of exposure light transmitted through transmitting portions adjacent to each other, so that high resolution cannot be obtained when an isolated bright line is to be formed.
On the other hand, according to an exposure method using the phase shift mask of the Halftone type described above, resolution of an isolated bright line can be improved. Therefore, if the masks of these two types can be fabricated on a same mask substrate, densely formed lines, an isolated bright line, and an isolated dark line of high resolution can simultaneously be obtained.
A mask structure and a method of manufacturing the mask structure for achieving the effect above are proposed in Japanese Patent Laying-Open Nos. 7-168342 and 6-123961. Phase shift masks disclosed in the Japanese Patent Laying-Open Nos. 7-168342 and 6-123961 are hereinafter described as a first conventional example and a second conventional example, respectively.
FIG. 13 is a cross sectional view schematically showing a structure of a phase shift mask according to the first conventional example. With reference to FIG. 13, a transparent substrate 201 is provided with a trench 201a, providing first and second light transmitting regions Tn and Ta having phases of light shifted by 180.degree. to each other. Between the first and the second light transmitting regions Tn and Ta, a semi-shading region S is produced by forming a semi-shading film 203 which covers a sidewall of trench 201a.
In a phase shift mask 210, a phase shift effect of the Halftone type is obtained by providing semi-shading film 203 having a width Ls greater than a prescribed dimension, and a phase shift effect of the Levenson type is achieved by providing semi-shading film 203 having the width Ls smaller than a prescribed dimension. By appropriately setting the width Ls of semi-shading film 203, both of the phase shift portion of the Levenson type and that of the Halftone type can be produced on the same mask.
A method of manufacturing the phase shift mask is hereinafter described.
FIGS. 14-17 are schematic cross sectional views showing a method of manufacturing the phase shift mask according to the first conventional example, following the order of process steps. With reference to FIG. 14, a chromium film 205 is deposited on a surface of a quartz substrate 201 and a resist pattern 207 is formed on chromium film 205. Chromium film 205 is patterned by anisotropic etching using resist pattern 207 as a mask. Resist pattern 207 is thereafter stripped.
Referring to FIG. 15, quartz substrate 201 is anisotropically etched using chromium film pattern 205 as a mask. A shifter pattern is transferred onto the substrate by providing trench 201a on the surface of quartz substrate 201. Chromium film pattern 205 is thereafter removed.
With reference to FIG. 16, the entire surface of quartz substrate 201 is exposed by the removal of chromium film pattern 205.
Referring to FIG. 17 next, a chromium film 203 is formed on the entire surface and a resist pattern 209 is formed on chromium film 203. Chromium film 203 is anisotropically etched using resist pattern 209 as a mask. Chromium film 203 is thus patterned exposing the first and second light transmitting regions while being left on the semi-shading region sandwiched between the first and second light transmitting regions. Resist pattern 209 is thereafter stripped and phase shift mask 210 illustrated in FIG. 13 is completed.
FIG. 18 schematically shows a cross section of a structure of the phase shift mask according to the second conventional example. With reference to FIG. 18, a phase shift mask 310 is provided with a phase shift mask portion of the Levenson type and that of the Halftone type. An etching stop film 303 is provided on an entire surface of a quartz substrate 301.
At the phase shift portion of the Levenson type, a shift layer 305 divided into a plurality of pieces is formed on etching stop film 303. A shading film 307 is formed on an edge portion of phase first layer 305 such that it partially exposes the surface of phase shift layer 305.
On the other hand, at the phase shift portion of the Halftone type, phase shift layer 305 divided into a plurality of pieces is formed on etching stop film 303. Semi-shading film 309 is provided such that it covers an entire surface of phase shift layer 305.
As clearly illustrated in FIG. 18, both of the phase shift portion of the Levenson type and that of the Halftone type are fabricated on the same mask.
A method of manufacturing the phase shift masks will be described.
FIGS. 19-23 are cross sectional views schematically showing the method of manufacturing the phase shift mask according to the second conventional example, following the order of process steps. First with reference to FIG. 19, etching stop film 303, phase shift layer 305, and shading film 307 are successively formed on quartz glass substrate 301. A resist pattern 311a is deposited on shading film 307. Shading film 307 is anisotropically etched using resist pattern 311a as a mask. Resist pattern 311a is thereafter removed.
Referring to FIG. 20, a plurality of pieces of shading film 307 separated from each other on phase shift layer 305 are formed at the phase shift portion of the Levenson type through this etching.
Referring to FIG. 21, semi-shading film 309 is formed to cover the entire surface of shading film 307 and phase shift layer 305.
Referring to FIG. 22, a resist pattern 311b having a prescribed shape is deposited on semi-shading film 309. Semi-shading film 309 and phase shift layer 305 are successively etched using resist pattern 311b as a mask. Resist pattern 311b is thereafter stripped. With reference to FIG. 23 next, a resist pattern 311c is further formed such that it covers the phase shift portion of the Halftone type. Semi-shading film 309 of the phase shift portion of the Levenson type is etched using resist pattern 311c as a mask and thereafter removed, thereby completing phase shift mask 310 shown in FIG. 18.
According to the first and the second conventional examples described above, a phase shift mask provided with a phase shift portion of the Levenson type and that of the Halftone type can be obtained.
However, in the first conventional example, the formation and patterning of semi-shading film 203 are carried out after the formation of trench 201a as shown in FIG. 17. In order to ensure satisfactory function of a phase shift mask of the Levenson type, semi-shading film 203 should be formed to exactly cover a sidewall of trench 201a which is a boundary portion of the regions having different light phases from each other. A problem of this example is that a highly precise alignment between semi-shading film 203 and an underlying shifter pattern is required when semi-shading film 203 is patterned.
In the second conventional example, phase shift layer 305 and semi-shading film 309 should be formed separately at the phase shift portion of the Halftone type as shown in FIGS. 19 and 21. A problem in this example is that the number of film formation steps increases and the process of manufacturing becomes complex, resulting in higher cost of manufacturing.
In the step of manufacturing a mask, a defect must be removed after patterning each film. However, in the second conventional example, removing the defect becomes difficult because of the increased number of film formation steps.
Furthermore, in the first conventional example, chromium film 203 is formed after the formation of the shifter pattern at quartz substrate 201 as shown in FIG. 16. When the shifter pattern is formed at quartz substrate 201 or after the formation, it is highly likely that a foreign matter is trapped in trench 201a. If chromium film 203 is formed having the foreign matter trapped as shown in FIG. 17, chromium film 203 could be broken due to the foreign matter. This leads to a problem that chromium film 203 tends to have a defect.
In the second conventional example, semi-shading film 309 is formed after the patterning of shading film 307 as shown in FIGS. 20 and 21, so that semi-shading film 309 is also likely to have a defect as in the first conventional example.